Metals, Crystals, Processes Flashcards
What three lightweight metals are used in aircraft?
Aluminium and Aluminium Alloys
Titanium and Titanium Alloys
Magnesium and Magnesium Alloys
What is the basic building block of a crystal called?
Unit Cell
How do Unit Cells form a crystal?
Repetition
Three types of crystals and their amount of atoms?
BCC - Body Centred Cubic - 9 Atoms
FCC - Face Centred Cubic - 14 Atoms
HCP - Hexagonal Close Packed - 17 Atoms
What is a Ferrous metal?
Contains iron
What is a Non-Ferrous metal?
Doesn’t contain iron
Steel Characteristics, Properties, and Identification
Base material is iron
Alloyed primarily with carbon
Molten iron alloyed with less than 2% carbon and poured into a mould
Cast iron is formed
How to make steel?
Pure iron is remelted in a special furnace where carbon is introduced along with other alloying elements to achieve the desired characteristics
What does Carbon do?
Primary hardening, permits heat treatment
Low Carbon Steels…
0.1% - 0.3% carbon
Medium Carbon Steels…
0.3% - 0.5% carbon
High Carbon Steels…
0.5% - 1.05% carbon
What does Sulphur do?
Normally kept low, decreases ductility
What does Phosphorous do?
Strength, hardness, corrosion resistance
What does Nickel do?
Hardenability and impact strength
What does Chromium do?
Corrosion resistance, high temperature strength
What does Molybdenum do?
Impact strength, creep strength
What does Vanadium do?
Tensile strength, yield strength
What does Titanium do?
Toughness
What are the Alloying Ingredients for Steel?
Carbon Sulphur Phosphorus Nickel Chromium Molybdenum Vanadium Titanium
What is a material designation?
Four digit code, first two denoting the steel and what Alloys have been used, the last two denotes the percentage of carbon (AISI or SAE numbers)
SAE 1030 (1 carbon steel, 0 plain carbon steel, 0.3% carbon)
What is Tempering?
Method used to decrease the hardness, thereby increasing the ductility and toughness of the quenched steel. Quenching produces stresses - tempering relieves these stresses
How to Harden Steel
Heated above it’s critical temperature, allowing carbon to disperse in the iron matrix
The alloy is then cooled rapidly by quenching it in water, oil, or brine
If the quench is too quick, the structure becomes martensitic (the hardness it could possibly be - too brittle for use on aircraft)
Which product gives the strongest quench?
Brine
What is Annealing?
Softens and relieves internal stresses
How to carry out Annealing
Heated to Upper Critical Limit + 50F
Soaked at this temperature
Cooled very slowly at room temperature
Results in very soft and ductile metal
Which process results in a very soft and ductile metal?
Annealing
How to carry out Normalising
Heat steal to 100 degrees F above its Upper Critical Limit
Soak for prescribed time
Cool at room temperature
What is Quenching?
The rate steel will cool down when quenched is governed by the quenching medium
What is Ferrite?
Pure iron
What is Austenite?
Can absorb more carbon (up to 2%)
What is Cementite?
Very very hard compound made of iron and carbon
What is Pearlite?
Structure composed of alternating layers of Ferrite and Cementite
What does CRES stand for?
Corrosion Resistant Stainless Steel
What is CRES?
Strength and toughness and resistance to high temperature
Identified by a three digit system 200,300,400
200 and 300 known as Austenitic
400 known as Ferrite
300 is the most common in aircraft use
Disadvantages of CRES
Difficult to cut
Greater coefficient of expansion
Not suitable in high temp environments (engines)
Experienced technician needed
What is Austenitic?
Chromium/Nickel and chromium nickel manganese alloy.
Face centred cube
18% or more carbon
3.5% - 22% nickel
Contains stabilising alloys
200/300 series
Cold worked/strain hardened
Non-magnetic
What is Ferritic?
Body centred cubic
400 series
15%-30% chromium
Small amounts of aluminium
Non-heat treatable (no Carbon)
Magnetic
What is Martensitic?
400 series
12%-18% chromium
High carbon content
Heat treatable
Least corrosion resistant
What is Precipitation Hardening?
Little carbon
15%-17% chromium
4%-7% nickel
Solution heat treated
Can be hardened to very high strengths
Excellent corrosion resistance
What is High Strength Low Alloy Steel?
Iron based alloys
Can be hardened to very high strengths
High strength alloy 300M used on landing gear
What is Hydrogen Embrittlement?
Phenomenon that occurs in Ferrous and Titanium alloys
The hydrogen migrated to an area such as the grain boundaries or voids in the metal. At high temperatures, hydrogen concentrations react with carbon to form methane which causes a build up of pressure that can cause cracks.
Name Steel Alloys Rework Precautions
Stress concentrations: Misuse of tools leading to scratches, general damage to surface finish, etc
Hydrogen embrittlement: Ensure all precautions are taken to prevent embrittlement
Name some Surface Protection
CRES doesn’t normally require protective treatment. Cadmium plated, zinc sprayed or chromate will prevent galvanic corrosion on light alloys.
Non Stainless Protection: Cadmium Paint Aluminium spray Chromium and Nickel plating Silver playing (not when in contact with Aluminium)
What is Galvanic Corrosion?
Galvanic Corrosion potential is a measure of how dissimilar metals will corrode when placed against each other in an assembly
What is Case Hardening?
Gives steel components a hard, durable surface that is suitable for load bearing whilst retaining a tough core
Not suitable for non-ferrous materials
What is Carburising?
Produces a thin layer of high carbon steel on the surface of a low carbon steel component
Heat metal to 900C while in contact with carbon-rich medium. Followed by suitable heat treatment
Three types of Carburising?
Pack
Gas
Liquid
What is Pack Carburising?
Heating a component up to temperatures of 1700F whilst it is packed into a carbon-rich material such as charcoal. Carbon penetrates the surface of metal causing it to harden
Metal then quenched in oil
What is Gas Carburising?
Object placed in basket in a furnace, through which a suitable carbon-rich gas is passed (e.g methane)
What is Liquid Carburising?
Object heated to a suitable temperature and immersed in a hot salt bath at 900C
What is Flame Hardening?
Surface heated to above Upper Temperature by an oxyacetylene torch and immediately quenched with a jet of water
What is Induction Hardening?
Similar to flame hardening, but the whole circumference is heated simultaneously by an induction coil
What is Nitriding?
Produces an improved final surface finish compared to carburising as the component is hardened, tempered, and ground to its final dimensions prior to surface hardening
What is Hardening?
Involves heating the material to 1000F in an Ammonia-rich environment. The ammonia breaks down, alloying the nitrogen to penetrate the surface. Special alloys, known as Nitroalloys, are used in the manufacture of compounds that are going to be used in the nitriding process
